in Electricity and Magnetism. 



311 



restrial magnetism) is not as the tangents of these arcs, as 

 would be the case if the deflective action of the helix on the 

 poles of the needle were constant, for all positions of the 

 needle, in amount as well as direction ; but depends on the 

 shape of the helix and the proportional length of the magnet 

 placed within ; because the action depends on the law, that 

 every portion of an electric current constantly tends to deflect a 

 magnetic pole, in a direction at a right angle to the current it- 

 self, and also at a right angle to the straight line from that pole 

 to the current, and with a force inversely as the square of the 

 distance of the pole from that portion of the current. 



In the case of a square helix and a magnet nine-tenths of 

 the average exterior and interior length of a side of the helix, 

 the tangential force arising from the deflection by the helix 

 varies with the angle of deflection, as here stated. 



Degrees of the are 



Tangential de- ~\ 



fleetive force J 



5° 

 074 



10° 

 0-56 



20° 

 0-36 



30° 45o 

 0-236 0112 



70° 

 002 



90° 

 000 



I found a useful and accurate form of galvanometer for 

 measuring and comparing the power of galvanic batteries, or 

 currents which are not very feeble, to be, a helix and a thin 

 flat magnet, so placed on an axis as to be deflected vertically 

 when a current is transmitted, and weights applied in any 

 way so as to retain the magnet in a horizontal position : the 

 weights should act not far from the axis, so as to make the 

 deflecting force more sensible by its greater leverage. The 

 helix wire should be thick and as short as possible ; three coils 

 are enough, so as not to impede the current to be measured. 



One cell on DanielPs constant principle, one foot high, and 

 two inches and a half broad, with a zinc rod inserted nine 

 inches in the liquid, and charged with a strong solution of 

 sulphate of copper, and 7 X diluted ordinary sulphuric acid, 

 at about 65° of Fahrenheit, and with the circuit quite com- 

 pleted, evolves a quantity of electricity due to the consump- 

 tion of 0*04' grain of zinc per second. For cells of such a 

 size, cylinders of wood prepared as in Mullins's battery can 

 easily be turned less than one-tenth of an inch thick and ten 

 inches long ; they do not appear to impede the current much 

 more than the gut used by Daniell, and are more durable and 

 easy to manage. 



When the cells are near 32° temperature the sulphate of 

 zinc as it is formed consolidates on the zinc rod ; although the 

 zinc be well amalgamated and quite pure, and the solution 

 quite fresh, the electric current acts by starts instead of being 

 perfectly uniform, as it is at 60° or even at 50°. 



